202 THE PRESENT EVOLUTION OF MAN — PHYSICAL 



Zymotic diseases, both in man and brutes, are due to 

 the presence of living ferments, minute animals or 

 vegetables, very low in the scale of life, which per- 

 manently or temporarily take up their abode in the 

 bodies of higher animals, drawing thence their sus- 

 tenance, and, either by themselves or by their poisonous 

 secretions, causing death or disablement, which latter 

 may be more or less complete and more or less 

 permanent, and which, therefore, may more or less 

 completely and permanently place the individual at- 

 tacked by them among the ranks of the unfit. In this 

 category are included all infectious and contagious 

 diseases, small-pox, cholera, scarlatina, measles, influ- 

 enza, syphilis, &c. In some of them the organism, 

 which causes the death or disablement, has been seen 

 and studied under the microscope, as, in tuberculosis 

 and cholera ; in all the others we are able by analogy to 

 infer with certainty the existence of pathogenic germs, 

 as in small-pox and measles. 



Concerning any disease prevalent in any given 

 area — e. g. scarlatina in England — experience teaches 

 that some individuals of the community are much more 

 susceptible to its action than are others. For instance, 

 in England, where scarlatina is prevalent, there are 

 people so constituted that they take the disease in a 

 severe form and perish of it, whereas there are others 

 who are totally immune to its action, or who take it in 

 so mild a form that they do not suffer more than a 

 passing inconvenience. Now it is manifest that, in the 

 presence of such a disease, those individuals who are 

 liable to be attacked by it in its severer forms are at 

 a disadvantage, and tend to be eliminated and to leave 

 no offspring, or if by chance they leave offspring, these, 

 inheriting the peculiarities of their parents, tend in 

 turn to be eliminated ; whereas those individuals who 

 are immune against the disease, or are so constituted 



